NASA recently announced that the Voyager 1 spacecraft has crossed the heliopause (the theoretical boundary where the Sun's solar wind is separated by the interstellar wind) and has entered interstellar space. (Credit: NASA/JPL-Caltech)

Spacecraft Voyager 1 has entered interstellar space, although exactly when it left the heliosphere is still up for debate.

“This is the first manmade object that has left our home—our bubble—ever,” says Merav Opher, associate professor of astronomy at Boston University and a guest investigator on NASA’s Voyager team. “Voyager is like our scout, telling us what lies beyond our home.”

NASA’s recent announcement, based on a new study published in the journal Science, follows a debate among some astronomers as to when or even whether the transit beyond the heliopause had occurred. The Science study places the transit as completed on August 25, 2012.

This artist’s concept puts solar system distances in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. One AU is the distance from the sun to the Earth, which is about 93 million miles or 150 million kilometers. Neptune, the most distant planet from the sun, is about 30 AU. NASA’s Voyager 1, humankind’s most distant spacecraft, is around 125 AU. It will take about 300 years for Voyager 1 to reach the inner edge of the Oort Cloud and possibly about 30,000 years to fly beyond it. (Credit: NASA/JPL-Caltech)

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In August, Opher, as part of a team of plasma physicists at the University of Maryland’s Institute for Research in Electronics and Applied Physics, published a paper maintaining that Voyager had crossed into interstellar space in July 2012.

Opher and plasma physics colleagues Marc Swisdak and James F. Drake of the University of Maryland constructed a model of the outer edge of the solar system that indicates Voyager 1 entered interstellar space a little more than a year ago.

Other recent papers had suggested the spacecraft was still in the heliosphere, a fuzzily defined transition zone between our sun’s sphere of influence and the rest of the galaxy.

Solar vs. galactic particles

At issue was what the boundary crossing should look like to Earth-bound observers 11 billion miles (18 billion kilometers) away. The sun’s envelope, known as the heliosphere, is relatively well understood as the region of space dominated by the magnetic field and charged particles emanating from our star.

However, the heliopause transition zone is both of unknown structure and location.

Conventional wisdom held that researchers would know when Voyager passed through this mysterious boundary when they stopped seeing solar particles and started seeing galactic particles, accompanied by a detected change in the prevailing direction of the magnetic field.

The UMD-BU model indicates that the Voyager 1 data can be consistent with entering interstellar space in 2012.

In describing on a fine scale how magnetic field lines from the sun and magnetic field lines from interstellar space can connect to each other based on the large-scale behavior, they conclude Voyager 1 has been detecting the interstellar magnetic field since July 27, 2012.

Other models envisioned the interstellar magnetic field draped around our solar bubble and predicted that the direction of the interstellar magnetic field would be substantially different from the solar magnetic field inside.

By that interpretation, Voyager 1 would still be inside our solar bubble.

“Now those scientists have reconsidered their position and are saying we are in the interstellar space, based on the density inferred from the radio data detected on Voyager,” says Opher. (There is no instrument that directly detects the density on Voyager 1.)

“So, we did cross the heliopause, although exactly when this happened is still in dispute.”